Icon Fonts: How do they work?

I understand that icon fonts are just fonts and that you can get the icons by just calling their classname, but how do icon fonts work?

I've tried checking the related icon font resources loaded in Chrome to see how icon fonts display icons (in comparison to general fonts) but I haven't been able to figure out how this happens.

I've also been unsuccessful in finding resources on how this "icon font technique" is done, even though there are loads of icon fonts available. There are also loads of resources showing how icon fonts can be integrated, but no one seems to be sharing or writing about how this is done!

Glyphicons are images and not a font. All the icons are found within a sprite image (also available as individual images) and they are added to the elements as positioned backround-images:

Actual font icons (FontAwesome, for instance) do involve downloading a specific font and make use of the content property, for instance:

@font-face {
    ...
    src: url('../font/fontawesome-webfont.eot?#iefix&v=3.0.1') format('embedded-opentype'),
         url('../font/fontawesome-webfont.woff?v=3.0.1') format('woff'),
         url('../font/fontawesome-webfont.ttf?v=3.0.1') format('truetype');
    ...
}

.icon-beer:before {
    content: "\f0fc";
}

As the content property isn't supported in older browsers, these also make use of images.

Here's an example of completely raw FontAwesome in use as a font, turning  ( - you may not be able to see this!) into an ambulance: http://jsfiddle.net/GWqcF/2

If your question is how a CSS class can insert a specific character (that will be rendered as an icon in the special font), take a look at the source for FontAwesome:

.icon-glass:before { content: "\f000"; }
.icon-music:before { content: "\f001"; }
.icon-search:before { content: "\f002"; }
.icon-envelope:before { content: "\f003"; }
.icon-heart:before { content: "\f004"; }

So a CSS content directive is used to insert the character (which is from a special private-use reserved area of Unicode that does not mess up other readers).

How webfont icons work?

Web-fonts icons work by using CSS to inject a specific glyph into the HTML using the content property. It then uses @font-face to load a dingbat webfont that styles the injected glyph. The upshot is that that injected glyph becomes the desired icon.

To begin, you’ll need a web-font file with the icons you need, either defined for particular ASCII characters (A, B, C, !, @, #, etc.) or in the Private Use Area of the Unicode font, which are spaces in the font that will not be used by specific characters in a Unicode encoded font.

Read more, how to create webfont icon at Responsive Webfont Icons.